KIEV/ODESSA, Ukraine (Reuters) - A young activist who became prominent in Ukraine's 2014 uprising and was later appointed to work on a major project to reform the corruption-plagued customs of Odessa is herself being investigated for corruption.

Roman Nasirov, the head of Ukraine's Fiscal Service, which is spearheading the probe, told Reuters that he suspected 27-year-old Yulia Marushevska, who resigned last month, of undervaluing cargo and other violations that he didn't specify.

He did not say whether other people besides Marushevska were being investigated.

As evidence, Nasirov told Reuters that, in the weeks since Marushevska stepped down on Nov 14, the Odessa region had increased its customs revenues by 30 percent, or 300 million hryvnia ($11 million).

He did not specify whether he was personally leading the investigation into Marushevska's actions. Reuters could not independently verify the figure and has no independent evidence of Marushevska's wrongdoing.

Nasirov's office did provide official data for customs revenues between January and October, which showed that Odessa had not met its revenue target for 7 out of the 10 months in 2016.

Marushevska's office has previously said that weaker revenues were due to the fact that some corrupt businesses avoided Odessa because of the reforms she had brought, thereby bringing in less revenue.

In a separate interview with Reuters, Marushevska denied all wrongdoing and blamed Nasirov himself for derailing her efforts to rid Odessa port and other nearby ports of endemic corruption.

She says she clamped down on the practice of goods being deliberately undervalued and accused Nasirov of shielding corrupt officers.

She called Nasirov a guard dog for a corrupt system.

"(The investigation) doesn't make any sense," she said. "It just demonstrates that this system isn't ready to change."

Nasirov denied her accusations as "hollow".

Their rift is part of a wider spat between reformers such as Marushevska who have been sacked or quit their positions this year and factions in the Western-backed Ukrainian authorities who took power after a pro-Kremlin president was toppled by protests in 2014.

Both the United States and the European Union say Ukraine must reform to honor the sacrifices made during the Maidan protests in which around 100 demonstrators were killed in the final days.

PUBLIC FEUD

Since Maidan there have been no notable corruption-related convictions and the country still ranked 130th out of 168 countries in Transparency International's corruption perceptions index in 2015.

The International Monetary Fund warned in October a $17.5 billion bailout program risked getting derailed if Kiev failed to show "concrete results" in tackling corruption. It also withheld a third tranche of the money for nearly a year and reduced its size to $1 billion from $1.7 billion citing Ukraine's "policy implementation".

Cleaning up Odessa was meant to be a major step, both real and symbolic, to secure the death of old Ukraine, which in the eyes of the reformers had been squeezed by a rotten state in collusion with oligarchs.

Marushevska was appointed to oversee customs clearances at several ports clustered around the city of Odessa, including Youzhny, Ukraine's largest port by volume, and Odessa port, the second largest.

Marushevska had no previous relevant experience when she took on the role in late 2015. Soon her time at the helm descended into feuding between her and Nasirov at press conferences and on social media.

Marushevska accused Nasirov of blocking her attempts to fire corrupt officials. A group of regional customs officers wrote a letter calling for her resignation, saying she had politicized the customs service.

"The main reason for me to go was that I came to the point where the change I started couldn't become sustainable because of the non-existent political will to do that," she said.

She also told Reuters that corruption had returned to Odessa customs since she resigned, an accusation also made by some local businessmen and Odessa's acting governor Solomiia Bobrovska.

"Today, businesses must again pay a bribe in addition to the official price. The businesses are talking about this publicly. This indicates that the customs are returning to the past," Bobrovska told Reuters but did not elaborate further.

Nasirov disputes that corruption is on the rise, calling it a "fable".